This invention relates to a printing method and system whereby the information printed on a substrate, such as paper, is transferred in the form of a latent image or "secure image" which is invisible to the eye and any other usual image detecting device at the time of printing and is revealed only after the substrate is subjected to a subsequent process of image activation.
Invisible inks have existed for many years. Many such ink systems have been found, developed and used in a limited way mainly because of the limited accessibility of such invisible inks and delivery systems for the latter.
A problem with the use of latent image printing for the purpose of product identification, to date, is that no good means has existed for placing such information on a variety of substrates. Contact printing is not suitable for many such uses as there is no good means to place the image on the desired substrate if it is irregularly shaped or is substantially inaccessible to traditional contact printing devices.
Thus, latent image, variable information such as production date, lot number, batch number, serial number, and the like, cannot presently be placed on many products, except by hand, which is cumbersome, expensive, and prone to inaccuracies, defeating the purpose of using such information.
Ink jet printing and ink compositions which permit ink to be jetted from an ink jet printer have never been used for the purpose of printing latent images, such as variable production information. Ink jet printing is presently employed for printing many types of visible images, using specially formulated inks. Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited.
Ink jet printing is a non-impact technique for projecting droplets of ink onto a substrate. There are two major categories of ink jet printing, "Drop-On-Demand" ink jet and "Continuous" ink jet. Using Drop-On-Demand ink jet technology, the ink is normally stored in a reservoir and delivered to a nozzle in the print head of the printer. A means exists to force a single drop of ink out of the nozzle whenever it is needed to print a single spot on the printed medium (for example, paper). For Continuous ink jet, a conducting ink is supplied under pressure to an ink nozzle and forced out through a small orifice, typically 35 to 120 .mu.m in diameter. Prior to passing out of the nozzle, the pressurized ink stream proceeds through a ceramic crystal which is subjected to an electric current. This current causes a piezoelectric vibration equal to the frequency of the AC electric current. This vibration, in turn, generates the ink droplets from the unbroken ink stream. The ink stream breaks up into a continuous series of drops which are equally spaced and of equal size. Surrounding the jet, at the point where the drops separate from the liquid stream is a charge electrode. A voltage is applied between the charge electrode and the drop stream. When the drops break off from the stream each drop carries a charge proportional to the applied voltage at the instant at which it breaks off. By varying the charge electrode voltages at the same rate as drops are produced it is possible to charge every drop to a predetermined level. The drop stream continues its flight and passes between two deflector plates which are maintained at a constant potential, typically .+-.2.5 kV. In the presence of this field, a drop is deflected towards one of the plates by an amount proportional to the charge carried. Drops which are uncharged are undeflected and collected into a gutter to be recycled to the ink nozzle. Those drops which are charged, and hence deflected, impinge on a substrate traveling at a high speed at right angles to the direction of drop deflection. By varying the charge on individual drops, the desired pattern can be printed.
The ink jet process is adaptable to computer control for high speed printing of continuously variable data. Ink jet printing methods can be divided into three general categories: high pressure, low pressure and vacuum techniques. All have been described and employed in conventional ink jet printing and can be employed in the present invention.
Reviews of various aspects of conventional ink jet printing can be found in these publications: Kuhn et al., Scientific American, April, 1979, 162-178 and Keeling, Phys. Technol., 12(5), 196-303 (1981). Various ink jet apparatuses are described in U.S. Pat. Nos. 3,060,429, 3,298,030, 3,373,437, 3,416,153 and 3,673,601.
German Pat. Specification No. 3,047,884 discloses the preparation of printed circuit boards employing an ink jet printer. Also disclosed is the spraying of organometallic solutions such as organocopper compounds directly onto an unmetallized circuit board. By means of a subsequent laser beam, the conductor pathways can be cured completely.
Vest et al., Int'l J. Hybrid Microelectronics, 6, 261-267 (1983), discloses computer controlled ink jet printing of hybrid microelectronics circuits. An ink jet printer is used with a conductor ink based on metallo-organic compounds in solution. The use of silver neodecanoate as a silver conductor is disclosed, with or without added platinum in the form of platinum amine octoate, to produce a solderable connector. Silver conductor line patterns on glass and alumina substrates were produced, the silver inks decomposed to silver when heated to 250.degree. C.
In general, an ink jet composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components and wettability of the substrate. Further, the ink must be quick-drying and smear resistant, must be capable of passing through the ink jet nozzle without clogging, and must permit rapid cleanup of the machine components with minimum effort.
Thus, ink jet printing has never been successfully used to print latent images.